Extraoral dental radiographic apparatuses can produce two kinds of radiographies: bi-dimensional (2D) radiographies, or volumetric (3D) radiographies.
In the field of bi-dimensional radiographies, panoramic radiographies and teleradiographies are included.
Panoramic radiography (also known as orthopantomography) produces a radiographic image of a curved plan, known as Welander's curve, approximating a patient's jaws, with blurring of the anatomical structures laying outside a narrow layer around the predesigned curved plane. This technology has been known since the '50s.
Teleradiography is a projective radiographic technique, producing radiographic images of the skull or of other anatomical areas from different projections, with minimum magnification and geometrical distortion. Usually two perspectives are represented, latero-lateral and anteroposterior.
Cone beam volumetric radiography (also known as CBCT) is the acquisition, from different projection angles, of a series of bi-dimensional radiographic images which will be processed post-acquisition to reconstruct three-dimensional volumes.
Extraoral radiographies are performed through known apparatuses, wherein typically at the two ends of a C-arm an X-ray source and an X-ray sensor are positioned, respectively, while a patient is placed between them. Typically the X-ray source and sensor rotate around patient's head, which is kept still by a positioning device (craniostat). The movements that mechanical parts have to perform in order to get this result is called trajectory.
In order to get a good image, the patient must remain still during the acquisition time, that is during X-ray emission. On the other hand, it is easy to realize that the patient will stand still if she/he is comfortable, while if she/he is uncomfortable, or under stress, there are more probabilities for her/him to move.
Another important point is that the craniostat should be as transparent as possible to X-rays, in order not to create artifacts which might reduce the diagnostic validity of the radiographic image. This has an impact on materials (no metallic materials can be used), but also on thicknesses, in that with increasing thickness even a radiolucent material begins to appear in radiographies. However, the reduction of thickness bears the risk of making the craniostat not rigid enough to ensure the solidity of the object and therefore the patient's immobility.
Moreover, as there is physical contact between patient and immobilizing device, the contact parts of the craniostat must be disposable, or must be disinfectable between patients.
For safety reasons it is important that electric parts (cables, motors, actuators, sensors) are not in the close proximity of the patient.
The craniostat must allow the positioning of any patient: from pediatric patients to adult patients of big dimensions (99 percentile), and therefore must fit cranial measures presenting a wide variability (cranium diameter going from 14 cm to 18.5 cm).
In order to perform all the kinds of acquisitions which will be described in the following, the extraoral dental radiographic apparatus must be able to perform an ample number of different trajectories, and therefore the C-arm needs wide possibilities of movement around the patient. Trajectories have a generically circular or semi-circular shape, and it is therefore possible to define an “anti-collision cylinder” wherein neither X-ray source nor X-ray sensor can enter. Once defined an as small as possible anti-collision cylinder, both the patient's head and craniostat must be contained inside it. It is therefore important that the craniostat is not bulky, so as to not constitute a limit for the trajectories.
In order to perform all the kinds of acquisition, especially teleradiographies, the craniostat has to be removed by X-ray path during the acquisition.
Finally, the craniostat must allow a rapid and easy positioning of the patient by the professional human operator.
To define the correct positioning of the patient's head, referring to anatomical portions and also to virtual planes which are defined on the basis of anatomical portions is necessary. The main and most widely known is Frankfurt plane, which is a virtual plane passing through the inferior margin of the left orbit and the upper margin of each ear canal or external auditory meatus, immediately under the eye.
In the family of panoramic radiographies there are different kinds of acquisitions, which are all obtained through blurring of anatomic structures outside a limited layer around a predefined plane. Each of this kind of acquisition has specific requirements for positioning the patient. To recapitulate only the main kinds of acquisitions:
Panoramic radiographies or orthopantomographies: the patient is usually oriented with her/his Frankfurt plane placed horizontally and in a sagittally symmetric position with respect to the acquisition trajectory. The two dental arches must not be overlapping as in the normal occlusal position, but to avoid overlapping in the image, they must be vertically pulled away on the same plane, in order to be sure that they are in the focal trough.
Latero-lateral Temporomandibular Joints (TMJ): in this case the focal plane sagittally cuts the condyle head. The preferred positioning is always with the Frankfurt plane placed horizontally. In order to have an effective diagnosis, often it is necessary to repeat consecutively the same acquisition, one with open mouth, one while the patient is occluding, without repositioning the patient.
Frontal or postero-anterior Temporomandibular Joints (TMJ): in this case the focal plane coronally cuts the condyle head. The preferred positioning is with the plane passing from the upper margin of the ear canal to the superciliary arch positioned horizontally (the head is inclined forward). In order to have an effective diagnosis, often it is necessary to repeat consecutively the same acquisition, one with open mouth, one while the patient is occluding, without repositioning the patient.
Bite-wing: this is a projection very similar to panoramic projection, but instead of acquiring dental arches in their complete height, the image is limited to the coronal portion of teeth. The patient must be placed as in the panoramic acquisition, but it being a very restricted field, the bite must be at a pre-defined height.
In three-dimensional acquisitions all the field of view is reconstructed, so it is important that the anatomic portion is analyzed is in the centre of the field of view. Especially for small fields of view (around 5 cm high), it might be important to align the patient with the mandibular plane placed horizontally, the mandibular plane ideally passing from chin end to mandible angle. In this way possible metallic artefacts remain on the plane of dental crowns, and especially third molars are on the same plane of incisors.
In a typical extraoral apparatus, the number of different acquisition programs is around thirty; all these programs are variations of the main kind of programs recapitulated above. A well-made craniostat should allow the acquisition of all the kinds of images necessary for diagnostic aims.